The Case of the Magnetic Mount

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bjnolley
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The Case of the Magnetic Mount

Post#1 » Mon Dec 19, 2016 12:49 pm

On 15 May 2006 Terry Cannon said:
The adjusting head on both DirecWay and StarBand dishes is steel and will affect a compass that is close enough. Often times, there is enough steel in an RV or nearby truck or auto to affect a compass reading.

I generally will take my compass reading from behind the dish. I'll try it at about 10' back, then move back another 5'-10', and do the same thing again. If all the compass readings agree, I think I'm in the ball park.

If I'm the least bit unsure of the magnetic compass reading, I'll start my satellite search 10°-20° degrees off to the left of where I think the satellite should be. As long as I’ve done a good setup and have a nice plumb mast and have set my elevation accurately, I’ll find that satellite on the first clockwise sweep (viewed from above).

Note: The new HughesNet Ka-band mounts are also steel, so this is still relevant for these systems also.

On 13 Jan 2007 Rayhound posted his method for dealing with this, normally without a compass:
I might use my compass once a year or so, when I may have trees or something in my line of sight.

As normally done by most folks, I set my elevation and skew on the dish and make sure my mast is plumb.

To this old land surveyor, I really do not care what the exact mag bearing is or exactly where it is. VBG

Let me explain that. Even "IF" (not impossible but highly unlikly) you could set in the exact mag bearing that is NOT really what you are after..... You are after the highest signal strength you can get from your satellite. The mag bearing is "just a calculated idea" of where you SHOULD get that best reading, which may or may not be true because of many factors.

So I just eyeball roughly where my mag bearing is supposed to be (from a line drawn on a computer mapping program, I use Delorme Mapping), and then use my trusty OPI meter (the predecessor to the DAPT meters) as a compass to get my highest satellite signal reading as I rotate my dish thru the mag bearing whatever it is. That is what I want, Signal Strength. I could care less what the exact mag bearing is.

Briefly, that is my method, which has been working for me for several years. Now I do freely admit that I sometimes have better days than others getting my signal. Last fall, in the middle of no where in northern Nevada, down in a twisty canyon, before dark but I could not see the sun, I pulled over for the night and set up my dish. Now if I would have used my compass it would have saved me about 15-20 minutes of daylight. That is about how long it took me to realize I was about 180º out of whack. OOPS!

Here is some basic info, to help understand:
    A circle has 360°. With a compass 0° points to the magnetic north pole. Azimuth is the same as a magnetic bearing. If your needed azimuth is 167º, this means you want a magnetic bearing of 167º, or to put it a different way, a clockwise angle of 167º from the 0º bearing of the magnetic north pole.

    When your compass is set up your compass 0º (north) will be pointing towards the magnetic north pole, sight from the center of the compass needle clockwise to the 167º mark on the compass circle and that will be your azimuth for 167º.
I hope this helps, but if does not I suggest you go to this website and study the lessons there.
http://www.learn-orienteering.org/old/
Note: Rayhound is no longer with us. He was an avid RVer who traveled in an old bus and he loved his satellite Internet!

Val added the following to the discussion:
As a retired AF Navigator and former Director of the Fels planetarium in Philadelphia, I used some of my expertise in setting up my satellite dish. What I have done precludes the use of a magnetic compass and only requires the sun as a means to pinpoint the azimuth of the satellite assigned to me. It will work for any other stationary satellite in the sky. This is what I have done.
    1. First, I determine my latitude and longitude using Microsoft's streets and trips.

    2. I run the programs developed by AL-Software which gives me the azimuth of the sun for every minute of the day. I print out only the time period needed to set up my dish.

    3. I installed a moveable compass rose around the base of my dish and a a pointer attached to my dish's mount. So as I turn my dish, the pointer points to different azimuth readings on the compass rose.

    4. I installed a string from the horn to the top of my dish.

    5. I then align my dish to the sun and center the shadow of the string on the center of my dish. The mast must be perfectly vertical and horizontally set and the skew must be at zero.

    6. When the shadow of the string is perfectly centered on my dish, I turn the compass rose to the azimuth of the sun at that precise time.

    7. Once this has been done, the compass rose must remain fixed as it is now oriented to true north.

    8. You can now turn your dish until the pointer reads the true azimuth of your satellite.

    9. You can now set the skew and altitude as needed.
    This method precludes errors induced from magnetic disturbances in the ground, errors in compass readings and basically is much more accurate. Unfortunately, it only works when the sun is out and visible from your setup location.
Fortunately, I had several compass roses that I have adapted to my setup. Perhaps others can find a source for finding compass perhaps at drafting stores or office supplies. If anyone desires photos, please write.

Trailscout said:
In order to eliminate or reduce compass inaccuracies which may result from the effect of metal in the dish and/or the vehicle located nearby, the following procedure might be considered if not already used.

Use the compass to locate or position oneself several feet from the dish but in direct alignment between the dish and the satellite. Then turn 180 degrees and use the compass to sight some distant object that is in alignment with the direction of the satellite.

Return to the dish and sweep the area around this distant object until the satellite is located.

Finally, return to the location/spot where the compass was used for alignment and observe any compass disagreements with the direction the dish appears to be actually pointing. If there is a difference, make note of this difference for future "guesstimations".
Take care,
Barb Nolley
HughesNet mobile dealer and installer
http://www.MobileInternetSatellite.com

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